Queues are used for three things building, recruiting and researching. These are vital to developing your city fast. There also comes strategy to it, do you want to go for building queue for fast leveling at start, or maybe a research queue which is fairly needed all through the game, or a recruiter which you will never stop needing.
Costs[]
Now they arn't giving these away for free (mostly), but the cost for a second queue in any category is 75 gold. For a third queue it is 150 gold.